This invention relates to apparatus for reducing the negative air pressure that normally exists behind a moving vehicle such as a truck or tractor-drawn trailer.
It is well-known that a moving vehicle develops a positive air pressure on surfaces which are presented in the direction of travel and negative or subatmospheric pressure (hereinafter called vacuum sometimes for the sake of brevity) simultaneously on the rear side of such surfaces. Thus, considering a rapidly moving vehicle as a whole, negative pressure normally exists behind its rear so there is a pressure differential or force that tends to subtract from the force and energy provided by the engine for driving the vehicle forward. The fact that significant amounts of snow are drawn into the ambient around the rear of a truck and plastered on its rear surface is some manifestation of the existence of these differential pressure conditions. Of course, the forces required to compensate or equalize the pressure differentials must be derived from the engine at the expense of increased fuel consumption.
Improving the aerodynamic properties of vehicles, particular commercial vehicles, which travel great distances, is a widely recognized requirement. Many proposals have been made for fulfilling this requirement. Some give attention to reducing air resistance of the tractor or truck cabin and to the body of the vehicle and others position airfoils or deflectors at supposedly strategic locations in an effort to reduce negative pressure or air turbulence at some places near the front of the vehicles such as around the top and rear of the cabin.
By way of example, U.S. Pat. No. 3,929,202 to Hobbensiefken basically teaches sloping the front upper part of a truck or tractor cabin rearwardly and placing angulated airfoils or deflectors in the front end region of the vehicle including a deflector forwardly of the frontmost wheels above road level to reduce turbulence and possibly vacuum conditions near the front end of the vehicle, but it is not evident that the problem of reducing vacuum at the rear of the vehicle as a whole is specifically addressed.
U.S. Pat. No. 4,119,339 to Heimburger illustrates a deflector mounted forward of the front wheels and of the engine for diverting air flow downwardly and sideways when the vehicle is in motion. A mechanism is provided for enabling the deflector to be pivoted upwardly to a more nearly horizontal position to get it out of the way when there is a prospect for driving over very rough terrain which could result in deflector destruction. The deflector is not located where it could have significant effect on reducing vacuum at the rear of the vehicle nor does it seem that the problem is recognized.
U.S. Pat. No. 3,348,873 to Saunders teaches locating airfoils or deflectors in the front end region of a tractor and van type trailer combination mainly in the region defined by the rear of the tractor cabin and the front of the van body. One deflector is ahead of the rear wheel of the tractor but there is no suggestion for locating a deflector where it might be most effective to reduce vacuum or negative pressure which is prevalent in the rearmost region of the vehicle and which has the greatest effect on increasing the engine load.